This invention relates generally to communication systems and more particularly, it relates to a channel unit interface circuit for interconnecting a two-wire transmission path to a four-wire transmission path. The channel unit interface circuit has particular use in direct current (DC low frequency, and voice frequency applications for interfacing between a multiplexer having a four-wire circuit and a customer station having a two-wire circuit.
Typically, burglar and fire alarm companies use two-wire transmission lines leased or rented from telephone companies for connecting alarm-monitoring equipment located in their monitoring office to alarm-detection equipment at a customer's premise. These two-wire lines are generally a part of the same wire pairs in a large multiconductor cable furnished by telephone companies over the past many years. However, in recent years, the telephone companies have generally not been burying or hanging additional cables to be used by the alarm companies but have been converting the existing cables to a digital carrier system in order to obtain an increased number of voice and data transmissions over a single cable.
As a result, a new problem has been created since most of the alarm signals generated by the burglar and fire alarm companies are simply not compatible with existing digital carrier channel units which are the normal interface with the digital carrier system. At present, there are at least fifty different alarm techniques which are utilized in signalling alarm conditions over the two-wire cables and only a small number of them can be directly coupled into the digital carrier system with the existing channel units. While some alarm companies have adapted their particular alarm signals so as to operate with such existing channel units, this suffers from the disadvantage of requiring the alarm or telephone companies to design special equipment for use with each of the different alarm techniques, which becomes quite complicated and uneconomical.
Hence, there has arisen a need of a channel unit interface circuit which is compatible with substantially all of the different alarm techniques. In the present invention, there is provided a channel unit interface circuit which simulates a cable pair or transmission lines over a digital carrier system in the frequency range of DC to 3.5 KHz with an amplitude between +110 to -200 VDC, or any amplitude range.